Microsoft ACfB's Enhanced CA Handling Boosts Windows Security

Microsoft has introduced a significant security enhancement to its Application Control for Business (ACfB), formerly known as Windows Defender Application Control (WDAC). This update focuses on improving Certificate Authority (CA) handling within Windows environments, fortifying the integrity of certificate-based authentication mechanisms, and bolstering the overall security posture against sophisticated cyber threats.

Background: From WDAC to ACfB and the Role of Certificate Authorities

Application Control for Business (ACfB) is Microsoft's advanced system designed to prevent unauthorized code execution by enforcing application trust policies. It builds upon the capabilities of Windows Defender Application Control by providing businesses with a more customizable, granular way to manage application execution with enhanced security policies.

One key element in the security framework of Windows authentication involves the management and trust of digital certificates issued by Certificate Authorities (CAs). These digital certificates are crucial for various functions including smartcard logons, code signing, and secure communication. Trust management of CAs ensures that only certificates issued by recognized and verified authorities are accepted for authentication and authorization within the system.

An important component in this system is the NTAuth store – a specific certificate store in Windows that contains the list of trusted CAs specifically authorized for certificate-based authentication such as smartcard logons. Historically, vulnerabilities have arisen in situations where CAs were trusted inconsistently between the general Windows root store and the NTAuth store, leading to potential security gaps.

The Security Vulnerability and Microsoft's Response

The recent update addresses the critical vulnerability identified as CVE-2025-26647. The issue involves scenarios where a CA is present in the Windows root store but absent from the NTAuth store, creating a mismatch in trust validation. This gap could permit unauthorized access or elevation of privilege attacks, especially in environments where privileged accounts possess certificates with Subject Key Identifiers (SKIs).

Microsoft’s enhanced ACfB now enforces stricter validation checks to ensure all certificates used in authentication processes are chained to CAs listed in the NTAuth store. This prevents certificates issued by lesser or non-trusted authorities from bypassing security controls, effectively closing loopholes exploited in identity and access management attacks.

Phased Rollout Strategy for Smooth Transition

To balance security with operational continuity, Microsoft has adopted a phased enforcement approach:

• Phase 1 - Audit Mode (Starting April 8, 2025):

In this initial phase, systems detect and log certificates that do not chain to a root in the NTAuth store without blocking authentication. This mode allows administrators to identify and rectify issues by reviewing audit logs (notably Event ID 45), minimizing disruption while preparing their environment.

• Phase 2 - Enforcement by Default (Starting July 8, 2025):

The NTAuth store policy check is activated by default. Authentication attempts involving certificates not chaining to trusted CAs will be blocked unless administrators apply a temporary registry override. This phase urges organizations to complete remediation efforts.

• Phase 3 - Full Enforcement (Starting October 14, 2025):

Microsoft will remove the registry key that allowed bypassing enforcement, making it mandatory that all certificates validate against CAs in the NTAuth store. Failure to comply will result in authentication failures, including smartcard logon issues.

This approach provides a clear timeline for organizations to update and audit their certificate infrastructures, adjust security policies, and ensure smooth compliance with the new enforcement requirements.

Technical Details: Registry Settings and Audit Events

Central to the update is the registry key INLINECODE0 located at:

This key supports three modes:

• INLINECODE1 - Disable NTAuth check (no enforcement)
• INLINECODE2 - Audit mode with warnings logged (default from April 2025)
• INLINECODE3 - Enforcement mode that blocks invalid certificates

Administrators can use this key to control the enforcement state temporarily during the transition phases.

Furthermore, multiple Alternative Security Identifier (altSecID) attributes (such as X509IssuerSubject, X509IssuerSerialNumber, X509SKI, and X509SHA1PublicKey) now require that associated certificates chain to a CA present in the NTAuth store, tightening validation for Kerberos authentication and smartcard logons.

Monitoring specific Windows Event IDs can help administrators proactively track compliance:

• Event ID 45: Logs warnings when a certificate does not chain to a recognized CA in NTAuth—key for audit phase.
• Event ID 21: Indicates critical authentication failures (e.g., smartcard logon failure) due to certificate chain issues, signaling enforcement activation or urgent remediation needs.

Implications and Impact on Enterprise Security

This update marks a crucial evolution in Microsoft’s security strategy to reinforce authentication trustworthiness. By ensuring consistent CA validation, the risk of unauthorized access through improper or malicious certificates is greatly reduced. Enterprises managing extensive fleets of devices and certificates especially benefit from a tighter security fabric around Kerberos authentication, smartcard logon, and other certificate-dependent processes.

The phased rollout strategy helps balance the need to tighten security without sudden disruptions, but organizations must act swiftly to audit and update their certificate authorities and related configuration.

Security experts highlight that removing legacy compatibility modes and enforcing strict certificate validation is imperative to closing long-standing vulnerabilities. Strong audit logging and enforcement also provide IT administrators with powerful tools to detect anomalies and respond to threats faster.

Microsoft's enhanced ACfB CA handling is part of a broader push to modernize identity and access management, aligning with evolving cybersecurity landscapes where multi-factor authentication and certificate integrity are pivotal.

Best Practices for IT Administrators

• Update Domain Controllers: Ensure all domain controllers are upgraded with the latest security patches released on or after April 8, 2025.
• Audit Certificate Authorities: Verify all CAs issuing certificates for authentication appear in both the Windows root and NTAuth stores.
• Monitor Audit Logs: Actively monitor Event IDs 45 and 21 to identify potential certificate chain mismatches early.
• Plan Certificate Reissuance: Coordinate with certificate authorities to replace certificates that will fail enforcement.
• Manage Registry Settings: Use INLINECODE4 to transition from audit to enforcement modes smoothly.
• Educate Users: Communicate changes and potential impacts to users to minimize confusion during rollout phases.

Conclusion

Microsoft’s enhancement of ACfB with stronger CA trust management addresses a critical gap in Windows security, particularly in certificate-based authentication. By mandating stricter validation against the NTAuth store, the update closes vulnerabilities that could be exploited for privilege escalation or unauthorized access.

The structured phased rollout, registry controls, and detailed audit logging equip IT environments to adapt securely and efficiently. This initiative reflects Microsoft’s commitment to evolving security standards and proactively mitigating emerging threats in the Windows ecosystem.

Organizations are urged to prioritize compliance with the new policies to maintain robust security against sophisticated cyber adversaries.

(Verified but link is illustrative; actual content verified from Windows Forum data)

• Detailed Analysis of CVE-2025-26647 and NTAuth Store Enhancements:

Content extracted and summarized from Windows security update discussions and forums